* Air Force aims for mid-2016 start
* Navy eyes mid-2018 date for operational use of F-35s
* Marines sticking to mid-2015 target date (Adds reaction, F-35B vertical takeoff)
The U.S. Air Force plans to start operational use of Lockheed Martin Corp.-built F-35 fighter jets in mid-2016, a year earlier than planned, using a similar software package as the Marine Corps, two sources familiar with the plans said on Monday.
The Air Force's decision to accelerate its introduction with a slightly less capable version of the F-35 software package means the planes will carry fewer weapons at first, although the software will later be upgraded to the final version, said the sources, who were not authorized to speak publicly.
Air Force spokeswoman Ann Stefanek said a final decision had not been made and declined to comment further. A spokesman for the Pentagon's F-35 program office declined to comment.
The decision reflects the military's desire to start using the new warplanes, which are already rolling off the assembly line at Lockheed's sprawling Fort Worth, Texas, plant, even as military officials continue to test the plane.
"This decision gets that (U.S.) fifth-generation capability out on the front lines that much sooner," said one of the sources familiar with the Air Force's plans. "It also sends a message about confidence in the program to Congress and the international partners."
Former Air Force Secretary Michael Wynne said accelerating operational use of the new warplane would allow the Air Force to learn more about the F-35's integrated battle management system.
"This is not just about replacing aging F-15's or F-16's; it is about changing the order of battle and truly embracing a integrated form of warfare where the F-35 manages the targeting and directs supporting fire at the same time as providing more precise aim points," Wynne told Reuters in an email.
The Air Force, Marines and Navy must report to Congress by June 1 on their target dates for initial operational capability, or IOC, which marks the point when the services have enough planes on hand to go to war if needed. Actual deployments usually lag IOC dates by about a year.
The sources said the services would send Congress a list of target or "objective" dates for declaring initial operational capability and a list of "threshold" dates, or deadlines.
The Marines Corps is sticking to its plan to begin early operational use in mid-2015 of its F-35B jets, which can take off and land like a helicopter. It will be the first of the three U.S. military services to start using the jets.
Its threshold is the end of 2015. The planes will run the F-35's 2B software, which will give the Marines an initial war fighting capability that includes some air-to-air skills, the ability to strike targets on the ground and carry several internal weapons, including laser-guided bombs.
Lockheed on Monday said one of its F-35 B-model planes completed the first-ever vertical takeoff on May 10, demonstrating a capability needed for repositioning jets in areas where they cannot perform a short takeoff.
The Navy has set mid-2018 for starting operational use of its C-model F-35, which is designed for use aboard U.S. aircraft carriers. Its deadline or threshold date is early 2019.
The Air Force decision marks a reversal from its earlier insistence that it needed the final 3F software package and comes after a Pentagon report cited China's development of two new fifth generation fighters over the past year.
The Air Force began studying the possible change several months ago. Lieutenant General Charles Davis told reporters in March that it might make sense to declare initial operating capability earlier than initially planned, given that the weapons on board would be suitable for basic war fighting needs.
The Air Force will have about 100 F-35s by 2016, when it plans to declare the planes ready for operational use.
The Pentagon's program chief, Lieutenant General Christopher Bogdan, told lawmakers last month he was "moderately confident" that the 2B software -- and the associated 3I software being developed for international buyers -- would be completed in time for the planned Marine Corps IOC in mid-2015.
The Air Force jets would use the 3I software, which will include a technology refresh with improved memory processors for some sensors on board.
Bogdan said it was not as clear that work on the final software package would be done in 2017, when the Air Force initially planned to declare IOC.
The final 3F software will support use of the aircraft's full war fighting capability, with additional internal and external weapons, and more advanced air-to-air and air-to-ground capabilities.